When I first entered Destiny Stone lvl 3 I somewhere - I think it was N18E19 - encountered a Magic Mouth or "A voice whispers:" that said something like "Your party can have no more than six characters.", my party had seven.
I clicked it away too fast and wanted to read it again but it didn't appear on the automap and after leaving end re-entering the room there was nothing. So I reloaded but then it doesn't even appear when entering the room for the first time and nowhere else in DS lvl3 outside the Snare.
Even reloading to when I first entered the Destiny Stone and completely mapping lvl1-3 (exept the Snare) doesn't make it appear again.
Anyone knows what happened?

I googled it and looked at some maps (not too carefully, because I don't want to spoiler myself) but no map has a place hinting at the number of chars in the party.

If you're playing the remaster (should specify, because there are some significant gameplay differences), then it's possible that this line may have been added in the remaster. Also, it may have been added post-release; I know they switched around the clue locations so that you don't unexpectedly enter the snare after the first one. (By the way, those arrow clues were originally in the manual.)

Furthermore, it's quite possible that the game may actually check the size of your party before displaying that message.

I'm playing BT2 from the remastered trilogy. Legacy mode, "Automap disabled" and "Save at guild only" unchecked.
The arrow clues are all there, only the message about party size appeared only once and I can't do anything to make it appear again. It's probably nothing game-changing but it drives me crazy that I saw it once and can't make it appear again, regardless of what I've done so far. I think it's either a bug or a random event (although I don't think there are random events in BT, excepts battles, of course).

If I enter the snare with seven chars, will I just die after running out of time and have no idea why or are there hints about party size within the snare?

There's an NPC who will offer to join your party; he won't join if you have a full party, unless one of those characters is a monster you can drop. There's some clues elsewhere (not in the snare) about this particular NPC.

Ok, thank you. I think i will just stop trying to make this hint appear again and continue with the snare.

Because it still bothered me what message it was I went through the game files with an hex-editor and i think I now know what happened:
Probably a monster wanted to join my party, I declined and got the message
"You have no space in your party for another monster. Next time, you are at the Adventurer's Guild you should leave a slot free for monsters to join..."
That's the only thing that comes close, no message about too many chars in the party exists in the DS section of the game files.